Gimbaled table riser support system

ABSTRACT

For a spar type floating platform having risers passing vertically through the center well of a spar hull, there is provided apparatus for supporting the risers from a gimbaled table supported above the top of the spar hull. The table flexibly is supported by a plurality of non-linear springs attached to the top of the spar hull. The non-linear springs compliantly constrain the table rotationally so that the table is allowed a limited degree of rotational movement with respect to the spar hull in response to wind and current induced environmental loads. Larger capacity non-linear springs are located near the center of the table for supporting the majority of the riser tension, and smaller capacity non-linear springs are located near the perimeter of the table for controlling the rotational stiffness of the table. The riser support table comprises a grid of interconnected beams having openings therebetween through which the risers pass. The non-linear springs may take the form of elastomeric load pads or hydraulic cylinders, or a combination of both. The upper ends of the risers are supported from the table by riser tensioning hydraulic cylinders that may be individually actuated to adjust the tension in and length of the risers. Elastomeric flex units or ball-in-socket devices are disposed between the riser tensioning hydraulic cylinders and the table to permit rotational movement between the each riser and the table.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation-in-Part of U.S. patentapplication Ser. No.09/677,814, filed Oct. 3, 2000, to which applicationpriority is claimed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to offshore mineral drilling andproduction platforms of the spar type and, more particularly, isconcerned with apparatus for supporting drilling and production risersfrom a gimbaled table supported above the top of the spar hull whereinthe table is compliantly constrained, but allowed limited rotationalmovement with respect to the spar hull. The Continuation in Partapplication is also concerned with an improved and simplified keel jointfor the spar type platform, and a yaw limiting apparatus for thegimbaled table.

[0005] 2. Description of the Prior Art

[0006] Drilling and production operations for the exploration andproduction of offshore minerals require a floating platform that is asstable as possible against environmental forces, even in severe weatherconditions. Among the six degrees of freedom of a floating platform, themost troublesome to drilling and production operations are the pitch,heave, and roll motions.

[0007] Present spar type floating platforms typically have drilling andproduction risers that are supported by means of buoyancy cans attachedto each of the individual risers. As the water depth in which a platformwill be used increases, the diameter and length of the buoyancy cansmust be increased to support the in-water weight of the risers and theircontents. Larger diameter buoyancy cans require larger spar center wellsizes, which in turn increases the spar hull diameter. Increasing thespar hull diameter and size in turn increases the hydrodynamicenvironmental loads acting on the spar. A larger size mooring system isthen required to withstand the increased environmental loads. The totalriser buoyancy can system for deep-water spar platforms can become verylong and heavy, significantly increasing the fabrication andinstallation costs.

[0008] With present spar platforms having a buoyancy can riser supportsystem, as the spar hull displaces laterally in response toenvironmental loads, the risers undergo a considerable amount ofdownward motion, or pull-down, with respect to the spar hull. Thisamount of riser pull-down increases as the water depth and riser lengthincreases, and requires longer jumper hoses, large clear verticalheights between the top of the hull and the drilling deck, andexpensive, large stroke keel joints.

[0009] Consequently, a need exists for improved apparatus for supportingdrilling and production risers from a spar type floating platform.Preferably, such an improved apparatus will eliminate the need for riserbuoyancy cans. It will preferably also reduce the amount of riserpull-down relative to the spar hull as the spar pitches and displaces inresponse to environmental forces. Such an improved riser supportapparatus will also preferably reduce the amount of fixed ballastrequired, reduce the need for, or length of, riser jumper hoses, andreduce the size and diameter of the spar hull. It will also preferablybe less expensive to build, install, and maintain than individual riserbuoyancy can systems in present use.

[0010] With respect to the Continuation in Part application, the keeljoint described in U.S. Pat. No. 5,683,205 to Halkyard for “StressRelieving Joint for Pipe and Method” consists in a guiding sleeve wherethe vertical riser passes through. The sleeve, by having rings at eachopen end for engagement with the riser, allows the sleeve to distributethe bending stress at two spaced areas on the riser. The sleeve is alsoprovided with wear means for contact with the keel. U.S. Pat. No.5,873,677 to Davies for “Stress Relieving Joint for Riser” consists in arotating keel joint having a ball joint fixedly attached to the keelopening. The sleeve of the riser is connected to the ball joint and wearmeans are provided between the sleeve and the riser.

[0011] However, there are several problems with keel joints of the priorart. First, they require long lengths of stress relieving sleeve. Priorart keel joints are also complex and expensive to build. Therefore, aneed exists for and improved and simplified keel joint for the spar typeplatform that does not require a lengthy stress-relieving sleeve.

BRIEF SUMMARY OF THE INVENTION

[0012] The present invention provides a riser support and tensioningapparatus and method and simplified keel joint that satisfies theaforementioned needs. According to one aspect of the invention, for aspar type floating platform having risers passing vertically through thecenter well of a spar hull, the spar hull having a top surface,apparatus is provided for supporting the risers from the spar hull. Theapparatus comprises a table disposed above the spar hull top surface anda plurality of non-linear springs associated with the table and the sparhull for permitting rotational movement between the table and the sparhull. The apparatus also comprises means for attaching the upper ends ofthe risers to the table.

[0013] According to another aspect of the invention, for a spar typefloating platform having risers passing vertically through the centerwell of a spar hull, the spar hull having a top surface, apparatus isprovided for supporting the risers from the spar hull. The apparatuscomprises a table disposed above the spar hull top surface. The tablecomprises a grid having openings therethrough. The risers pass throughrespective openings in the table grid. For each riser, at least oneriser tensioning hydraulic cylinder is provided, having one end attachedto the riser and the opposite end attached to the table, such that thetension in and length of the riser may be adjusted by operation of theriser tensioning hydraulic cylinder. A plurality of elastomeric loadpads are disposed between the table and the spar hull for permittingrotational movement therebetween. Larger capacity load pads are locatednear the center of the table for supporting the majority of the risertension, and smaller capacity load pads are located near the perimeterof the table for controlling the rotational stiffness of the spar hull.

[0014] According to a still further aspect of the invention, for a spartype floating platform having risers passing vertically through thecenter well of a spar hull, the spar hull having a top surface,apparatus is provided for supporting the risers from the spar hull. Theapparatus comprises a table disposed above the spar hull top surface.The table comprises a grid having openings therethrough. The risers passthrough respective openings in the table grid. For each riser, at leastone riser tensioning hydraulic cylinder is provided, having one endattached to the riser and the opposite end attached to the table, suchthat the tension in and length of the riser may be adjusted by operationof the riser tensioning hydraulic cylinder. A plurality of tablesupporting hydraulic cylinders is disposed between the table and thespar hull for permitting rotational movement therebetween. Each tablesupporting hydraulic cylinder has a first end pivotally attached to thetable and a second end pivotally attached to the spar hull. At least onelateral support shaft has an upper end pivotally attached to the tableand a lower end. For each lateral support shaft, at least one guide isattached to the spar hull for slidably receiving the lower end of thelateral support shaft.

[0015] According to another aspect of the invention, for a spar typefloating platform having risers passing vertically through the centerwell of a spar hull, the spar hull having a top surface, apparatus isprovided for supporting the risers from the spar hull. The apparatuscomprises a table disposed above the spar hull top surface. The tablecomprises a grid having openings therethrough. The risers pass throughrespective openings in the table grid. For each riser, at least oneriser tensioning hydraulic cylinder is provided, having one end attachedto the riser and the opposite end attached to the table, such that thetension in and length of the riser may be adjusted by operation of theriser tensioning hydraulic cylinder. A plurality of pedestals isprovided, each pedestal having a lower end attached to the spar hull andan upper end higher than the table for hanging the table therefrom. Foreach pedestal, at least one non-linear spring is associated with thetable, the pedestal, and the spar hull for permitting rotationalmovement between the table and the spar hull.

[0016] According to still another aspect of the invention, for a spartype floating platform having risers passing vertically through thecenter well of a spar hull, apparatus is provided for suspending andtensioning a riser from a surface associated with the spar hull, and forpermitting limited rotational movement between the riser and thesurface. The apparatus comprises a hydraulic cylinder having one endattached to the riser and the other end attached to the surface. Thetension in the riser may be adjusted by operation of the hydrauliccylinder. Means is provided for permitting rotational movement betweenthe riser and the surface.

[0017] According to still another aspect of the invention, a method isprovided for supporting a riser at a floating spar hull, the spar hullhaving a top surface. The method comprises the step of connecting atable to the spar hull, wherein the table has a limited range ofrotational movement with respect to the spar hull top surface inresponse to environmental forces acting on the spar hull. The methodfurther comprises the steps of suspending the riser from the table andof tensioning the riser.

[0018] With respect to the Continuation in Part application:

[0019] The present invention provides a keel joint for limiting bendingstresses in the risers at the keel. The keel joint comprises anelongated guide attached to the keel of the spar hull. The guide has avertical bore therethrough. A shaft is fitted within the bore of theguide. The shaft has a vertical bore therethrough for passage of one ofthe risers therethrough. A wear insert is associated with the shaft. Thewear insert has an outer surface for slidingly engaging a portion of thekeel joint.

[0020] The present invention also provides means for limiting yawmovement of the table. According to one aspect of the invention, themeans for limiting yaw movement comprises a yaw control shaft extendinghorizontally from the table. At least one spherical bearing is attachedto the yaw control shaft near its outer end. A pair of linear-sphericalbushings is disposed on opposite sides of the yaw control shaft andmated to the spherical bearing for limited rotation thereon. Structureis associated with the spar hull forming a guide slot. Thelinear-spherical bushings are disposed within the guide slot fortranslational movement therein. Means is also provided for limitingsurge and sway movements of the table with respect to the spar hull.

[0021] According to another aspect of the invention, the means forlimiting yaw movement of the table comprises a first pair of collinearguide shoes extending from opposite sides of the first end of the table.A second pair of collinear guide shoes extend from opposite sides of thesecond end of the table. The collinear axes of the first and secondpairs of guide shoes are laterally offset from the center of the table.A third and fourth pair of collinear guide shoes extend from oppositeends and opposite sides of the table. The collinear axes of the thirdand fourth pairs of guide shoes are positioned radially with respect tothe center of the table. For each guide shoe, a respective bearing plateis attached to the spar hull. The guide shoe abuts the bearing plate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0022] For a more complete understanding of the invention, and theadvantages thereof, reference is now mad to the following detaileddescription of the invention taken in conjunction with the accompanyingdrawings, in which:

[0023]FIG. 1 is a schematic, side elevation view in cross-section of aspar type floating platform having a riser support apparatus of thepresent invention.

[0024]FIG. 2 is a plan view of the riser support table of the presentinvention.

[0025]FIG. 3 is a side, cross-sectional view of an apparatus of thepresent invention for supporting and tensioning the risers.

[0026]FIG. 4 illustrates an alternative, ball-in-socket device that maybe used in the apparatus of FIG. 3.

[0027]FIG. 5 is a schematic, side elevation view in cross-section of theupper portion of the spar hull and an embodiment of the riser supportapparatus of the invention utilizing elastomeric load pads.

[0028]FIG. 6 is a schematic, side elevation view in cross-section of theupper portion of the spar hull illustrating an alternative embodiment ofthe invention utilizing table supporting hydraulic cylinders.

[0029]FIG. 7 is a schematic, side elevation view in cross-section of theupper portion of the spar hull illustrating an alternative embodiment ofthe invention wherein the riser support table is hanging from pedestalsattached to the spar hull.

[0030]FIG. 8 illustrates an embodiment of the invention utilizing bothelastomeric load pads and table supporting hydraulic cylinders.

[0031]FIG. 9 is a view, taken along the longitudinal center line andpartially in cross-section, of a first embodiment of a simplified keeljoint of the present invention that uses no sleeve.

[0032]FIG. 10 is a view, taken along the longitudinal center line andpartially in cross-section, of a second embodiment of a keel joint ofthe invention having a sleeve fitted around the wear insert.

[0033]FIG. 11 is a view, taken along the longitudinal center line andpartially in cross-section, of a third embodiment of a keel joint of theinvention having a more compact sleeve fitted around the wear insert.

[0034]FIG. 12 is a plan view of a riser support table equipped with afirst embodiment of a yaw limiting apparatus of the invention having akingpost.

[0035]FIG. 13 is an enlarged view of the encircled portion denoted “A”in FIG. 12.

[0036]FIG. 14 is an elevation view, taken along the centerline andpartially in cross-section, of the riser support table and yaw limitingapparatus of FIG. 12.

[0037]FIG. 15 is a plan view of a riser support table equipped with asecond embodiment of a yaw limiting apparatus of the invention having aplurality of guide shoes.

[0038]FIG. 16 is an elevation view, taken along the centerline andpartially in cross-section, of one side of the riser support table andyaw limiting apparatus of FIG. 15.

[0039]FIG. 17 is a plan view of a riser support table equipped with athird embodiment of a yaw limiting apparatus of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Referring now to the drawings, and more particularly to FIG. 1,there is schematically shown a side elevation view of a spar typefloating platform, generally designated 10, employing a riser supportapparatus of the present invention. Spar platform 10 includes spar hull12 having buoyancy tanks 14 at its upper end. Production risers 16 anddrilling riser 18 extend from wells (not shown) on the sea floor 20 upthrough keel joint 22 at the lower end of spar hull 12. The risers 16and 18 extend up through the center well 24 of spar hull 12 and are tiedat their upper ends to riser support apparatus 26. Riser supportapparatus 26 includes riser support table 28, which is compliantlysupported above top surface 30 of spar hull 12 by non-linear springs 32.Trees 34 are attached to the upper ends of risers 16 and 18. Spar hull12 floats at and extends slightly above water surface 36.

[0041] Referring now to FIG. 2, there is shown a plan view of risersupport table 28. Table 28 is made up of beams 38 interconnected to forma grid. Production risers 16 and drilling riser 18 pass throughrespective openings 40 of the grid of table 28.

[0042]FIG. 3 illustrates an apparatus of the present invention forsupporting and tensioning risers 16 and 18 from riser support table 28.As seen in FIG. 3, riser support bracket 42 is clamped or welded toriser 16 above table 28. Riser tensioning hydraulic cylinders 44 locatedbelow riser support bracket 42 have pistons 46 attached to riser supportbracket 42. The bottoms of hydraulic cylinders 44 are attached to table28 by elastomeric flex units 48. Elastomeric flex units 48 permitrelative rotation between hydraulic cylinders 44 and table 28, and thusbetween riser 16 and table 28. Some degree of rotation between risers 16and 18 and table 28 is necessary because risers 16 and 18 will tend toremain parallel to the axis of spar hull 12, or tilt with spar hull 12,as table 28 rotates relative to spar hull 12. Elastomeric flex unitsinclude rigid portions 50 and flexible portions 52 between rigidportions 50. Rigid portions 50 are preferably made of steel, andflexible portions 52 are preferably made of an elastomeric material.

[0043] After risers 16 and 18 are installed on table 28, hydrauliccylinders 44 may be operated to adjust the tension and lengths of therisers to provide the correct fixed ballast to the spar hull from theriser weight, and to compensate for temperature changes in the riserscaused by the produced fluid and the temperature of the surroundingrisers.

[0044]FIG. 4 illustrates an alternative device to elastomeric flex units48 for permitting relative rotation between hydraulic cylinders 44 andtable 28. In this embodiment, a segment of a ball 54 is attached to thebottom of hydraulic cylinder 44, and a mating cup 56 is attached totable 28. Spherically shaped surface 58 of cup 56 slidingly engages thespherical surface of ball segment 54, and permits relative rotationbetween hydraulic cylinder 44 and table 28, and thus between riser 16and table 28.

[0045]FIG. 5 illustrates a first embodiment of a riser support apparatusof the present invention. In this embodiment, elastomeric load pads 58and 60 function as non-linear springs 32 for compliantly supportingtable 28 above top surface 30 of spar hull 12, as described withreference to FIG. 1. Elastomeric load pads 58 and 60 are sized to bestrong enough to support the tension in all of the risers 16 and 18 andwith a spring rate that keeps the heave period of the spar platform andthe riser support system larger than the dominant wave period.Elastomeric load pads 58 and 60 are placed laterally around table 28 insuch a manner as to allow table 28 to rotate to a limited degreerelative to spar hull top surface 30 as spar hull 12 pitches in responseto environmental forces. This relative rotation is necessary to preventlarge axial tension and compression fluctuations in risers 16 near theouter perimeter of table 28. Risers 16 are axially secured at theirupper ends to table 28, and at their lower ends to the sea floor.Therefore, if table 28 were rigidly fixed in its position above sparhull top surface 30 without any means for relative rotationtherebetween, a tilt of spar hull 12 from its normally vertical positionwould induce large compressive loads in the risers 16 on the side ofspar hull 12 tilted down. This large compressive load would overstressand eventually buckle these risers. Similarly, the risers 16 on theopposite side of spar hull 12 would experience large tensile loads. Thelarge variations in axial tension and compression in risers 16 wouldresult in unacceptable fatigue damage to risers 16 over the lifetime ofthe installation. The relative rotation between table 28 and spar hull12 permitted by elastomeric load pads 58 and 60 allows the upper ends ofrisers 16 to “float” with respect to upper surface 30 of spar hull 12,and thus prevents large axial tension and compression fluctuations inrisers 16 resulting from environmentally induced pitching of spar hull12.

[0046] As seen most clearly in FIG. 2, large capacity elastomeric loadpads 58 are located near the center of table 28 for supporting a largeportion of the riser tension. Smaller capacity elastomeric load pads 60are located near the perimeter of table 28 for controlling therotational stiffness of table 28 with respect to spar hull 12. Thecombined axial stiffness of all the risers 16 and 18 installed on thespar platform varies in direct proportion to the number of risersinstalled. When fewer risers are installed, their combined axialstiffness is reduced proportionately. Therefore, the vertical stiffnessof the riser support apparatus does not normally require adjustment asrisers 16 and 18 are added to, or removed from, table 28. Furthermore,regardless of the number of risers installed on table 28, the heaveperiod of the spar platform and riser support system will be greaterthan the dominant wave period if the appropriate spring rate is chosenfor elastomeric load pads 58 and 60.

[0047] As additional risers are suspended from table 28, the rotationalstiffness of the riser support system may be increased by insertingadditional smaller capacity elastomeric load pads 60 around theperimeter of table 28. Alternatively, variable stiffness elastomericload pads may be used for load pads 60. These commercially availableload pads have an interior, sealed air chamber that can be pressurizedor depressurized as needed to adjust their stiffness.

[0048]FIG. 6 illustrates an alternative embodiment of a riser supportapparatus of the present invention. In this embodiment, table supportinghydraulic cylinders 62 and 63 function as non-linear springs 32 forcompliantly supporting table 28 above top surface 30 of spar hull 12 asdescribed with reference to FIG. 1. Large capacity hydraulic cylinders62 are located near the center of table 28 for supporting a largeportion of the riser tension. Smaller capacity hydraulic cylinders 63are located near the perimeter of table 28 for controlling therotational stiffness of table 28 with respect to spar hull 12. In orderto permit table 28 to rotate about both horizontal axes with respect tospar hull 12, the upper ends of hydraulic cylinders 62 and 63 arepivotally attached to table 28, and the lower ends are pivotallyattached to spar hull 12.

[0049] Air-over-oil accumulators 64 are hydraulically connected tosmaller capacity hydraulic cylinders 63 for providing them with anadjustable spring rate. For a stiff spring rate, a relatively smallamount of air should be maintained in accumulators 64. The use ofhydraulic cylinders 63 with air-over-oil accumulators 64 providesgreater operational flexibility than the riser support apparatus of FIG.5. Both the tension force and the stiffness of hydraulic cylinders 63can easily be adjusted over time by simply increasing or decreasing theair pressure in accumulators 64.

[0050] Because table supporting hydraulic cylinders 62 and 63 operate incompression and are hinged at their opposite ends, table 28 must belaterally supported with hydraulic cylinders 62 and 63 in their uprightposition to prevent table 28 and hydraulic cylinders 62 and 63 fromfolding down flat against upper surface 30 of spar hull 12. Lateralsupport shafts 66 provide the required lateral stability to the risersupport apparatus of FIG. 6. The upper ends of lateral support shafts 66are pivotally attached to table 28 so as to permit relative rotationbetween table 28 and spar hull 12. The lower ends of shafts 66 areloosely fitted within guides 68 attached to spar hull 12. Lateralsupport shafts 66 slide axially within guides 66 as table 28 tilts withrespect to upper surface 30 of spar hull 12 in response to environmentalloads. For a spar hull 12 having a center well 24 of squarecross-sectional shape, four lateral support shafts 66 are preferablyused, one being located near each of the four corners of center well 24.

[0051]FIG. 7 illustrates another alternative embodiment of a risersupport apparatus of the present invention. In this embodiment, table 28is partially supported from the bottom only by elastomeric load cells 58located near the center of table 28. To provide additional verticalsupport and the necessary lateral stability, table 28 is hung frompedestals 70. The lower ends of pedestals 70 are rigidly attached tospar hull 12, and their upper ends are higher than table 28 so thattable 28 may be hung therefrom. Table supporting hydraulic cylinders 63are used to provide limited rotational movement to table 28. With thisarrangement, table 28 is naturally stable because it is suspended froman upper support structure.

[0052]FIG. 7 illustrates two ways in which table 28 may be hung frompedestals 70 by hydraulic cylinders 63. The first way is illustrated atthe right end of table 28. Here, hydraulic cylinder 63 has an upper endpivotally connected to the top of pedestal 70 and a lower end pivotallyconnected to table 28, so that hydraulic cylinder 63 directly supportstable 28 from pedestal 70. Air-over-oil accumulator 64 is placed ontable 28 near, and is hydraulically connected to, hydraulic cylinder 63to provide it an adjustable spring rate as described above withreference to hydraulic cylinders 63 in FIG. 6.

[0053] The second way in which table 28 may be hung from pedestals 70 isillustrated at the left end of table 28. Here, pulley 72 is pivotallymounted near the top of pedestal 70. Cable 74 passes over the top ofpulley 72 and has one end attached to table 28 and the opposite endattached to the upper end of hydraulic cylinder 63. The lower end ofhydraulic cylinder 63 is attached to spar hull 12 so that the tension incable 74 is borne by hydraulic cylinder 63. Air-over-oil accumulator 64is placed on spar hull 12 near, and hydraulically connected to,hydraulic cylinder 63 as described above. Although not illustrated,hydraulic cylinder 63 could instead be mounted on table 28 and connectedto the opposite or right end of cable 74. In that case, the left end ofcable 74 opposite hydraulic cylinder 63 would be connected directly tospar hull 12.

[0054]FIG. 8 illustrates a combination of some of the above describedalternative embodiments of the riser support apparatus of thisinvention. Such a combination of features may provide the most desirablesystem in terms of operational flexibility. Large, rather stiffelastomeric load pads 58 placed under and near the center of table 28support the majority of the tension in risers 16 and 18. Four lateralsupport shafts 66 pivotally attached to table 28 and located near thecorners of center well 24 of spar hull 12 provide the needed lateralstability to table 28. Smaller capacity table supporting hydrauliccylinders 63 located under and near the perimeter of table 28 providethe proper rotation stiffness. Depending on the direction of rotation oftable 28, hydraulic cylinders 63 could act in either compression ortension. The tension and stiffness of hydraulic cylinders 63 can beadjusted by adjusting the air pressure in accumulators 64 to keep theoverall rotational stiffness of table 28 at the desired level over timeas wells are drilled and additional production risers 16 are installed.

[0055] A coupled computer aided design analysis was performed to comparea number of variable design parameters of a spar floating platformhaving a riser support system of the present invention with those of atraditional spar platform having risers individually supported bybuoyancy cans. The analysis was based on the following fixed designparameters for both types of spar platforms: Design Basis Water depth:4500 feet Topside weight: 39,000 tons Topside VCG above hull top: 80feet Wind sail area: 68,000 square feet Wind center of pressure: 150feet Number of wells: 20 Well pattern: 5 × 5 Production risers: outercasing outer diameter: 13.375 inches outer casing thickness: 0.48 inchesinner casing outer diameter: 10.75 inches inner casing thickness: 0.797inches tubing outer diameter: 5.5 inches tubing thickness: 0.415 inchesOuter casing design pressure: 4000 psi Inner casing design pressure:8500 psi Tubing design pressure: 8500 psi Fluid weights underproduction: Outer casing: 8.55 ppg Inner casing: 15.5 ppg Tubing: 5.5ppg Riser tree elevation: 55 feet Total riser weight at tree elevation:872 kips Riser weight at keel: 736 kips Riser wet weight per foot: 191lb/ft. Riser EA/L: 325 kips/ft.

[0056] The coupled design analysis resulted in the following designparameters for spar platforms having each type of riser support system:Traditional spar Spar with riser with riser support system buoyancy cansof invention Spar center well wet wet Center well size (feet) 75 × 75 50× 50 Spar hull diameter (feet) 158 150 Draft (feet) 650 650 Hard tankdepth (feet) 255 245 Freeboard (feet) 55 55 Truss height (feet) 360 380Soft tank height (feet) 35 25 Hull steel weight (tons) 29,937 29,200Fixed ballast (tons) 36,668 21,844 Riser tension supported (tons) 014,160 Variable ballast (tons) 12,347 14,398 Number of mooring lines 1616 Mooring pattern 4 × 4 4 × 4 Pretension (kips) 650 550 Fairleadelevation (feet) 255 245 Upper chain diameter (inches) 5.875 5.875length (feet) 250 250 Wire diameter (inches) 5.375 5.125 length (feet)6000 5500 Lower chain diameter (inches) 5.875 5.875 length (feet) 200200

[0057] There are several advantages attained by the use of the gimbaledtable riser support system of the present invention with a spar typefloating platform. First, the magnitude of spar pitch motions arereduced 10 to 25 percent from those of a traditionally designed sparwith buoyancy cans. Second, because the gimbaled table supports therisers, the riser weight replaces fixed ballast in the spar hull.Therefore, the amount of fixed ballast required is greatly reduced byapproximately 40 percent. Third, the need for buoyancy cans forsupporting the risers is eliminated. This also eliminates releasedbuoyancy can concerns and the need for buoyancy can guide structures.Fourth, riser pull-down relative to the spar hull is significantlyreduced, which reduces jumper hose requirements. Fifth, a simplifiedkeel joint design may be used. Sixth, the present invention permitseasier drilling and production operations and easier access to trees andrisers. Seventh, the riser tensioning system becomes more manageable andinspectable. Eighth, riser interference is essentially eliminated.Ninth, the spar hull diameter and center well size may be reduced. Thisin turn reduces the mooring line size requirement. Tenth, the smallersea floor riser pattern reduces the amount of lateral offset of the sparplatform. Eleventh, slip joint requirements are reduced, andrequirements for drilling tensionsers and workover riser tensioning areeliminated. Twelfth, special workover buoyancy requirements areeliminated. Thirteenth, the smaller size center well permits reducedtopside dimensions. Fourteenth, tensioning system redundancy is notrequired for each individual riser. Therefore, the need for an extrabuoyancy chamber in each riser is eliminated. Finally, a riser supportsystem of the present invention is less expensive to build, install, andmaintain than the individual riser buoyancy can system in present use.

[0058] With respect to the Continuation in Part application, thesimplified keel joint of the present invention is designed for use withsurface supported vertical risers (SSVR) on a spar type floatingproduction platform. The purpose of the keel joint is to limit thebending moment in the riser at the location where the riser enters thecenter well of the hull at the keel. As the floating platform moves inresponse to environmental conditions, the risers contact the hull bottomdue to the lateral offset of the hull relative to the fixed location ofthe risers on the seabed. This lateral offset also induces relativevertical movement between the hull and the risers. Additional relativemovement between the risers and the hull is generated due to the heaveresponse of the vessel. This relative movement between the risers andthe platform hull may cause contact wear that would be detrimental overthe life of the system.

[0059] The simplified keel joint consists mainly in a guide attached tothe keel of the spar hull, a single ball joint and, in some embodiments,a sleeve for contact wear. The keel joint segregates the functions ofrotation of the risers within the keel in response to bending moments onthe risers and wear in response to relative motion between the risersand the hull. This configuration allows the uses of specific materialsto minimize wear and galling at the ball joint and of standard vesselconstruction materials for the sleeve, in which a certain amount of wearcan be designed for and tolerated.

[0060]FIG. 9 is a view, taken along the longitudinal center line andpartially in cross-section, of a simplified keel joint 22 of the presentinvention. Keel joint 22 includes elongated guide 82 attached to thekeel of the spar hull (not illustrated). Keel joint 22 also includesshaft 86 contained within the vertical bore 84 of guide 82. Shaft 86 ismade up of a pair of tapered pipe sections 90 having flanges 92 on oneend. Flanges 92 are joined together end-to-end. A riser (notillustrated) passes through vertical bore 88 in shaft 86.

[0061] Ball wear insert 94 is attached to the outer circumferentialsurface of flanges 92. Ball wear insert 94 has an outer surface forslidingly engaging a portion of the keel joint 22. The convex outershape of ball wear insert 94 permits a small degree of rotation of shaft86 within guide 82. Ball wear insert 94 also absorbs contact wear withguide 82. In this embodiment, the ball joint comprises flanges 92, theirball wear insert 94, and the central portion 96 of guide 82. In oneembodiment, the diameter of bore 84 in guide 82 is 50 inches±¼ inches,and the outer diameter of ball wear insert 94 is 48 inches±{fraction(1/16)} inches.

[0062] Ball wear insert 94 slidingly engages a central portion ofelongated guide 82. The central portion of guide 82 engaged by wearinsert 94 has a thickened wall with respect to the wall thickness of theremainder of the guide 82 for withstanding stress imposed thereon bywear insert 94. The length of central portion 96 corresponds to thenormal stroke of the riser within keel joint 22. The length of guide 82corresponds to the expected extreme stroke of the riser. Guide 82 isdesigned for contact wear with ball wear insert 94.

[0063]FIG. 10 is a view, taken along the longitudinal center line andpartially in cross-section, of a second embodiment of a keel joint 98 ofthe invention. Keel joint 98 includes a sleeve 100 fitted within thebore 84 of the guide 82 and slidable therein. Sleeve 100 has a centralopening 102 therein containing wear insert 94 and at least a portion ofshaft 86. Sleeve 100 has an inner surface 104 slidingly mating to wearinsert 94 for permitting rotation of the riser and shaft 86 with respectto sleeve 100 and guide 82.

[0064] In the illustrated embodiment, wear insert 94 comprises a ballwear insert and the mating sleeve surface 104 is concave for conformingto the ball wear insert 94 shape. Sleeve 100 resists contact loadbetween shaft 86 and guide 82. Sleeve 100 is also designed for contactwear with ball wear insert 94. In one embodiment, the diameter of bore84 in guide 82 is 50 inches±¼ inch, and the outer diameter of sleeve 100is 48 inches±¼ inch.

[0065]FIG. 11 is a view, taken along the longitudinal center line andpartially in cross-section, of a third embodiment of a keel joint 106 ofthe invention. Keel joint 106 is similar in many respects to keel joint98 of FIG. 10. However, keel joint 106 has a more compact sleeve 108that fits closely around flanges 92 of pipe sections 90. Sleeve 108resists contact load between shaft 86 and guide 82 and is designed forcontact wear with ball wear insert 94. In one embodiment, a 1 inchnominal gap is provided between the outer surface of sleeve 108 and thebore wall of guide 82.

[0066] The invention also includes apparatuses 109 and 132 for limitingyaw movements of riser support table 28. The gimballing systemestablishes a center of rotation for the table about which it is allowedto roll and pitch (tilting movement of the spar) freely. This center ofrotation is also allowed to translate axially (heave movement of thespar) freely. That is, the table is allowed three degrees of freedomrelative to the spar hull: roll, pitch, and heave. Of the remainingthree degrees of freedom, two of them, the relative lateral translations(surge and sway) are eliminated except for minor gaps and elasticdeformations. The remaining degree of freedom (yaw) is eliminated by theyaw limiting device. The table gimballing system controls secondarilyinduced yaw movement of the table within acceptable limits. Thesecondarily induced yaw is a function of the table tilt angle and itsorientation with respect to the principal axes of the table.

[0067] FIGS. 12-14 illustrate yaw limiting apparatus 109 of the presentinvention. FIG. 13 is an enlarged view of the encircled portion denoted“A” in FIG. 12. Apparatus 109 includes a yaw control shaft 110 extendinghorizontally out from the table 28. As best seen in FIG. 13, a pair ofspherical bearings 112 are attached on opposite sides of yaw controlshaft 110 near its outer end. A pair of linear-spherical bushings 114are mated to respective spherical bearings 112 for limited rotationthereon. Linear-spherical bushings 114 have flat sides oppositespherical bearings 112 that slide against respective guide slot members116. Guide slot members 116 are fixed in position with respect to sparhull 12, and together form guide slot 118. Linear-spherical bushings 114are disposed within guide slot 118 for translational movement therein.

[0068] Means is also provided for limiting surge and sway movements ofriser support table 28 with respect to spar hull 12. Referring to FIG.14, kingpost shaft 120 has a lower end supported from spar hull 12 andan upper end near the center of table 28. Kingpost shaft 120 ispositioned coaxially with the central, longitudinal axis of spar hullcenter well 24. Kingpost shaft 120 is supported from spar hull 12 by abase pedestal 126. Pedestal 126 is secured to spar hull 12 or to asupport structure for table 28.

[0069] A linear-spherical inner bearing 122 slides axially alongkingpost shaft 120, and includes a spherical center portion 123. Aspherical outer bushing 124 is attached within an opening in risersupport table 28, and has a spherical inner surface that is mated tolinear-spherical inner bearing 122. The rotation of linear-sphericalinner bearing 122 within spherical outer bushing 124 permits risersupport table 28 to heave, pitch, and roll with respect to spar hull 12.In the illustrated embodiment, support cylinders 128 function asnon-linear springs 32. Support cylinders 128 may be pneumatic orhydraulic in various embodiments of the invention.

[0070] As seen in FIGS. 12-14, yaw limiting apparatus 109 offers noresistance to heave, roll, pitch, surge, or sway of riser support table28 with respect to spar hull 12. However, apparatus 109 does prevent yawmovement of table 28 about its vertical center line through the torquearm between kingpost 120 and spherical bearings 112 on shaft 110. Itshould be noted that yaw limiting apparatus 109 employs conventionalbearings that have either flat, cylindrical, or spherical bearingsurfaces that slide against similarly shaped surfaces of matingbushings. Therefore, the bearing areas are essentially independent ofthe bearing loads. Further, the bearing surfaces may be greased toimprove their bearing characteristics. The bearings may also be enclosedto retain the lubricant and for protection from environmental,contaminate, or mechanical damage. Additionally, the bearings haveinherently self-wiping edges that act as excluders of mechanical debrisand contaminates. This is particularly beneficial for the sphericalbearings 112 and linear-spherical bushings 114 that would be difficultto enclose.

[0071] One of the primary advantages of yaw limiting apparatus 109 isthat it uses conventional bearings that can be made in a machine shop toconventional tolerances. These are essentially unitized bearings thatcan be factory assembled and function tested before shipping. Further,they do not require sophisticated field assembly fit up that would berequired of unconventional bearings.

[0072] Another advantage of yaw limiting apparatus 109 is that all ofthe primary lateral table loads are taken out through base pedestal 126directly down into the table support structure. Therefore, the tablesupport structure may be raised up flush with the spar deck and theprimary lateral table loads are transferred out into the spar at thespar deck level. The secondary anti-yaw lateral loads are transferredout into the spar at the cellar deck level. This elevates the entireassembly of support cylinders up out of the center well and positionsthe support cylinders on the spar deck level where there would be goodaccess to them for installation, inspection, maintenance, repair, and/orchange-out. This also places the support cylinders in an environmentthat is inherently well ventilated.

[0073] FIGS. 15-16 illustrate a riser support table 28 utilizing yawlimiting apparatus 132 according to a second embodiment of the presentinvention. FIG. 15 is a plan view of table 28 equipped with yaw limitingapparatus 132. FIG. 16 is a partial elevation view taken along line16-16 in FIG. 15. The portion of table 28 omitted from FIG. 15 isidentical to that shown. Table 28 has first end 134 and second end 136.A first pair of collinear guide shoes 138 extend from opposite sides ofthe first end 134 of table 28. A second pair of collinear guide shoes140 extend from opposite sides of the second end 136 of table 28. Asseen in FIG. 15, the collinear axes of the first pair 138 and secondpair 140 of guide shoes is laterally offset an equal distance from thecenter of table 28.

[0074] A third pair of collinear guide shoes 142 extend from oppositeends 134 and 136 and from opposite sides of table 28. A fourth pair ofcollinear guide shoes 144 extend from opposite ends 134 and 136 and fromopposite sides of table 28. The collinear axes of the third pair 142 andthe fourth pair 144 of guide shoes is positioned radially from thecenter of table 28.

[0075] As best seen in FIG. 16, each guide shoe 138, 140, 142, and 144abuts a respective bearing plate 146 attached to spar hull 12. Bearingplates 146 comprise low friction material. Examples of such low frictionmaterial include molybdenum disulfide filled PTFE and carbon-graphitefilled PTFE.

[0076] Each guide shoe 138, 140, 142, and 144 comprises a base 148attached to table 28. Elastomeric cushion 150 is attached to the outersurface of base 148. A slide plate 152 overlies each elastomeric cushion150. Each slide plate 152 forms a segment of a horizontal circularcylinder in geometric shape.

[0077] Guide shoe pairs 138 and 140 located on the two opposite sides oftable 28 are positioned orthogonally with respect to the center of table28 so that they form segments of an imaginary horizontal circularcylinder enveloping these two sides of table 28. Guide shoes 138 and 140slide horizontally and vertically between two parallel, prismaticvertical walls formed by respective bearing plates 146 so as to resistsurge and yaw movements of table 28. Guide shoes 142 and 144 located onends 134 and 136, respectively, of table 28 are positioned radially withrespect to the center of table 28 so that they form segments or portionsof an imaginary sphere enveloping the orthogonal pair of sides of table28. Guide shoes 142 and 144 slide within vertical circular cylindricalwalls formed by respective bearing plates 146 so as to resist primarilysway movements of table 28, but not yaw movements. Therefore, yawlimiting apparatus 132 provides no over-constraint on yaw movements.Hence, there is no requirement for the guide shoes to have radialoffsets and/or excessive compliance. Table 28 is securely guided throughsecondarily induced yaw angular movements by apparatus 132 almost asprecisely as by yaw limiting apparatus 109 having the kingpostconfiguration (described above).

[0078]FIG. 17 is a plan view of a riser support table 28 equipped with athird embodiment of a yaw limiting apparatus of the invention in whichguide shoes 142 and 146 on each end of table 28 are spaced farther apartthan in the embodiment illustrated in FIG. 15.

[0079] The gimbaled table riser support system and method of the presentinvention, and many of its intended advantages, will be understood fromthe foregoing description of example embodiments, and it will beapparent that, although the invention and its advantages have beendescribed in detail, various changes, substitutions, and alterations maybe made in the manner, procedure, and details thereof without departingfrom the spirit and scope of the invention, as defined by the appendedclaims, or sacrificing any of its material advantages, the formhereinbefore described being merely exemplary embodiments thereof.

1. For a spar type floating platform having risers passing verticallythrough the center well of a spar hull, the spar hull having a topsurface, apparatus for supporting the risers from the spar hull, whichcomprises: a table disposed above the spar hull top surface; a pluralityof non-linear springs associated with the table and the spar hull forpermitting rotational movement between the table and the spar hull; andmeans for attaching the upper ends of the risers to the table.
 2. Theapparatus of claim 1, wherein the table comprises a grid having openingstherethrough, and wherein the risers pass through respective openings inthe table grid.
 3. The apparatus of claim 2, wherein the means forattaching the upper ends of the risers to the table comprises, for eachriser, at least one riser tensioning hydraulic cylinder having one endattached to the riser and the opposite end attached to the table, suchthat the tension in and length of the riser may be adjusted by operationof the riser tensioning hydraulic cylinder.
 4. The apparatus of claim 3,further including an elastomeric flex unit disposed between the risertensioning hydraulic cylinder and the table for permitting rotationalmovement between the cylinder and the table and thus between the riserand the table.
 5. The apparatus of claim 3, further including aball-in-socket device disposed between the riser tensioning hydrauliccylinder and the table for permitting rotational movement between thecylinder and the table and thus between the riser and the table.
 6. Theapparatus of claim 5, wherein the ball-in-socket device comprises asegment of a ball slidably disposed within a cup having a sphericallyshaped surface mating the ball segment.
 7. The apparatus of claim 1,wherein at least one of the nonlinear springs associated with the tableand the spar hull comprises an elastomeric load pad disposed between thetable and the spar hull.
 8. The apparatus of claim 1, wherein largercapacity non-linear springs are located between the table and the sparhull near the center of the table for supporting a large portion of theriser tension, and smaller capacity non-linear springs are locatedbetween the table and the spar hull near the perimeter of the table forcontrolling the rotational stiffness of the table.
 9. The apparatus ofclaim 1, wherein at least one of the non-linear springs associated withthe table and the spar hull comprises a table supporting hydrauliccylinder.
 10. The apparatus of claim 9, further including anair-over-oil accumulator connected to the table supporting hydrauliccylinder for providing an adjustable spring rate to the hydrauliccylinder spring.
 11. The apparatus of claim 9, wherein the tablesupporting hydraulic cylinder has a first end pivotally attached to thetable and a second end pivotally attached to the spar hull.
 12. Theapparatus of claim 11, further including at least one lateral supportshaft having an upper end pivotally attached to the table and a lowerend slidably attached to the spar hull.
 13. The apparatus of claim 12,further including at least one guide attached to the spar hull forslidably receiving the lower end of the lateral support shaft.
 14. Theapparatus of claim 12, wherein the center well of the spar hull issquare in cross-sectional shape, and wherein a lateral support shaft islocated near each of the corners of the center well.
 15. The apparatusof claim 9, further including at least one pedestal having a lower endattached to the spar hull and an upper end higher than the table, andwherein the table supporting hydraulic cylinder has a first endconnected to the table and a second end connected to the pedestal,whereby the table is hanging from the pedestal by the table supportinghydraulic cylinder.
 16. The apparatus of claim 9, further including: atleast one pedestal having a lower end attached to the spar hull and anupper end higher than the table; a pulley disposed near the top of thepedestal; and a cable passing over the pulley and having one endattached to the table supporting hydraulic cylinder and the opposite endattached to the table, whereby the table is hanging from the pedestal bythe cable, and whereby the cable tension is borne by the tablesupporting hydraulic cylinder.
 17. For a spar type floating platformhaving risers passing vertically through the center well of a spar hull,the spar hull having a top surface, apparatus for supporting the risersfrom the spar hull, which comprises: a table disposed above the sparhull top surface, the table comprising a grid having openingstherethrough, the risers passing through respective openings in thetable grid; for each riser, at least one riser tensioning hydrauliccylinder having one end attached to the riser and the opposite endattached to the table, such that the tension in and length of the risermay be adjusted by operation of the riser tensioning hydraulic cylinder;and a plurality of elastomeric load pads disposed between the table andthe spar hull for permitting rotational movement therebetween, whereinlarger capacity load pads are located near the center of the table forsupporting a large portion of the riser tension, and smaller capacityload pads are located near the perimeter of the table for controllingthe rotational stiffness of the spar hull.
 18. The apparatus of claim17, further including an elastomeric flex unit disposed between theriser tensioning hydraulic cylinder and the table for permittingrotational movement between the riser tensioning hydraulic cylinder andthe table and thus between the riser and the table.
 19. The apparatus ofclaim 17, further including a ball-in-socket device disposed between theriser tensioning hydraulic cylinder and the table for permittingrotational movement between the riser tensioning hydraulic cylinder andthe table and thus between the riser and the table.
 20. For a spar typefloating platform having risers passing vertically through the centerwell of a spar hull, the spar hull having a top surface, apparatus forsupporting the risers from the spar hull, which comprises: a tabledisposed above the spar hull top surface, the table comprising a gridhaving openings therethrough, the risers passing through respectiveopenings in the table grid; for each riser, at least one risertensioning hydraulic cylinder having one end attached to the riser andthe opposite end attached to the table, such that the tension in andlength of the riser may be adjusted by operation of the riser tensioninghydraulic cylinder; and a plurality of table supporting hydrauliccylinders disposed between the table and the spar hull for permittingrotational movement therebetween, each table supporting hydrauliccylinder having a first end pivotally attached to the table and a secondend pivotally attached to the spar hull; at least one lateral supportshaft having an upper end pivotally attached to the table and a lowerend; and for each lateral support shaft, at least one guide attached tothe spar hull for slidably receiving the lower end of the lateralsupport shaft.
 21. The apparatus of claim 20, further including anelastomeric flex unit disposed between the riser tensioning hydrauliccylinder and the table for permitting rotational movement between theriser tensioning hydraulic cylinder and the table and thus between theriser and the table.
 22. The apparatus of claim 20, further including aball-in-socket device disposed between the riser tensioning hydrauliccylinder and the table for permitting rotational movement between theriser tensioning hydraulic cylinder and the table and thus between theriser and the table.
 23. The apparatus of claim 20, wherein largercapacity table supporting hydraulic cylinders are located near thecenter of the table for supporting a large portion of the riser tension,and smaller capacity table supporting hydraulic cylinders are locatednear the perimeter of the table for controlling the rotational stiffnessof the table.
 24. For a spar type floating platform having riserspassing vertically through the center well of a spar hull, the spar hullhaving a top surface, apparatus for supporting the risers from the sparhull, which comprises: a table disposed above the spar hull top surface,the table comprising a grid having openings therethrough, the riserspassing through respective openings in the table grid; for each riser,at least one riser tensioning hydraulic cylinder having one end attachedto the riser and the opposite end attached to the table, such that thetension in and length of the riser may be adjusted by operation of theriser tensioning hydraulic cylinder; and a plurality of pedestals, eachpedestal having a lower end attached to the spar hull and an upper endhigher than the table for hanging the table therefrom; and for eachpedestal, at least one non-linear spring associated with the table, thepedestal, and the spar hull for permitting rotational movement betweenthe table and the spar hull.
 25. The apparatus of claim 24, wherein atleast one non-linear spring has a first end connected to the table and asecond end connected to the pedestal, whereby the table is hanging fromthe pedestal by the non-linear spring.
 26. The apparatus of claim 24,further including: a pulley disposed near the top of the pedestal; and acable passing over the pulley and having one end attached to thenon-linear spring and the opposite end attached to one of the spar hulland the table, whereby the table is hanging from the pedestal by thecable, and whereby the cable tension is borne by the non-linear spring.27. The apparatus of claim 24, further including a plurality ofelastomeric load pads disposed between the table and the spar hull forassisting the pedestals in supporting the table and risers.
 28. For aspar type floating platform having risers passing vertically through thecenter well of a spar hull, apparatus for suspending and tensioning ariser from a surface associated with the spar hull and for permittinglimited rotational movement between the riser and the surface, whichcomprises: a hydraulic cylinder having one end attached to the riser andthe other end attached to the surface, such that the tension in theriser may be adjusted by operation of the hydraulic cylinder; and meansfor permitting rotational movement between the riser and the surface.29. The apparatus of claim 28, wherein the means for permittingrotational movement between the riser and the surface comprises anelastomeric flex unit disposed between the hydraulic cylinder and thesurface.
 30. The apparatus of claim 28, wherein the means for permittingrotational movement between the riser and the surface comprises aball-in-socket device disposed between the hydraulic cylinder and thesurface.
 31. A method for supporting a riser at a floating spar hull,the spar hull having a top surface, the method comprising: connecting atable to the spar hull wherein the table has a limited range ofrotational movement with respect to the spar hull top surface inresponse to environmental forces acting on the spar hull; suspending theriser from the table; and tensioning the riser;
 32. The method of claim31, wherein the riser is tensioned by operating a hydraulic cylinderhaving one end attached to the riser and the opposite end attached tothe table.
 33. The apparatus of claim 1, wherein the spar hull includesa keel at the lower end of the center well, the risers passing throughthe keel, and further including a keel joint for limiting bendingstresses in the risers at the keel, the keel joint comprising: anelongated guide attached to the keel of the spar hull, the guide havinga vertical bore therethrough; a shaft fitted within the bore of theguide, the shaft having a vertical bore therethrough for passage of oneof the risers therethrough; a wear insert associated with the shaft, thewear insert having an outer surface for slidingly engaging a portion ofthe keel joint.
 34. The apparatus of claim 33, further including asleeve fitted within the bore of the guide and slidable therein, thesleeve having a central opening therein containing the wear insert andat least a portion of the shaft, the sleeve having a surface slidinglymating to the wear insert for permitting rotation of the riser and shaftwith respect to the sleeve.
 35. The apparatus of claim 34, wherein thewear insert comprises a ball insert and the mating sleeve surface isconcave for conforming to the ball insert shape.
 36. The apparatus ofclaim 33, wherein the shaft comprises a pair of pipe sections, eachsection having a flange on one end thereof, the flanges being joinedtogether end-to-end.
 37. The apparatus of claim 36, wherein the outersurfaces of the pipe sections are tapered.
 38. The apparatus of claim36, wherein the wear insert is attached to the outer circumferentialsurfaces of the flanges on the pipe sections.
 39. The apparatus of claim33, the wear insert slidingly engaging a central portion of theelongated guide, and wherein the central portion of the guide engaged bythe wear insert has a thickened wall with respect to the wall thicknessof the remainder of the elongated guide for withstanding stress imposedthereon by the wear insert.
 40. For a spar type floating platform havingrisers passing vertically through the center well of a spar hull, thespar hull having a top surface, apparatus for supporting the risers fromthe spar hull, which comprises: a table disposed above the spar hull topsurface; a plurality of non-linear springs associated with the table andthe spar hull for permitting rotational movement between the table andthe spar hull; means for attaching the upper ends of the risers to thetable; and means for limiting yaw movement of the table.
 41. Theapparatus of claim 40, wherein the means for limiting yaw movement ofthe table comprises: a yaw control shaft extending horizontally from thetable; as at least one spherical bearing attached to the yaw controlshaft near its outer end; a pair of linear-spherical bushings disposedon opposite sides of the yaw control shaft and mated to the sphericalbearing for limited rotation thereon; structure associated with the sparhull forming a guide slot, the linear-spherical bushings being disposedwithin the guide slot for translational movement therein; and means forlimiting surge and sway movements of the table with respect to the sparhull.
 42. The apparatus of claim 41, wherein the means for limitingsurge and sway movements of the table with respect to the spar hullcomprises: a kingpost shaft having a lower end supported from the sparhull and an upper end near the center of the table; a linear-sphericalinner bearing mounted on the kingpost and axially slidable thereon; aspherical outer bushing associated with the table and mated to thelinear-spherical inner bearing, whereby the table is permitted freedomof motion in heave, pitch, and roll with respect to the spar hull. 43.The apparatus of claim 42, wherein the kingpost shaft is supported fromthe spar hull by a base pedestal secured to the top surface of the sparhull.
 44. The apparatus of claim 41, wherein the kingpost shaft isdisposed coaxially with the longitudinal axis of the center well of thespar hull.
 45. The apparatus of claim 40, wherein the table has firstand second ends, and wherein the means for limiting yaw movement of thetable comprises: a first pair of collinear guide shoes extending fromopposite sides of the first end of the table; a second pair of collinearguide shoes extending from opposite sides of the second end of thetable, the collinear axes of the first and second pairs of guide shoesbeing laterally offset from the center of the table; a third pair ofcollinear guide shoes extending from opposite ends and opposite sides ofthe table; a fourth pair of collinear guide shoes extending fromopposite ends and opposite sides of the table; the collinear axes of thethird and fourth pairs of guide shoes being positioned radially withrespect to the center of the table; and for each guide shoe, arespective bearing plate attached to the spar hull, wherein the guideshoe abuts the bearing plate.
 46. The apparatus of claim 45, whereineach guide shoe comprises: a base attached to the table, the base havingan outer surface; an elastomeric cushion attached to the outer surfaceof the base; and a slide plate overlying the elastomeric cushion. 47.The apparatus of claim 46, wherein each slide plate forms a segment of ahorizontal circular cylinder in geometric shape.
 48. The apparatus ofclaim 45, wherein the bearing plates comprise low friction material.